1. Field of the Invention
The present invention relates to polypeptides having carboxypeptidase activity and isolated nucleic acid sequences encoding the polypeptides. The invention also relates to nucleic acid constructs, vectors, and host cells comprising the nucleic acid sequences as well as methods for producing the polypeptides. The present invention further relates to methods of obtaining protein hydrolysates useful as flavor improving agents.
2. Description of the Related Art
Various food products, e.g., soups, sauces and seasonings, contain flavoring agents obtained by hydrolysis of proteinaceous materials. This hydrolysis is conventionally accomplished using strong hydrochloric acid, followed by neutralization with sodium hydroxide. However, such chemical hydrolysis leads to severe degradation of the amino acids obtained during the hydrolysis, and also to hazardous byproducts formed in the course of this chemical reaction. Increasing concern over the use of flavoring agents obtained by chemical hydrolysis has led to the development of enzymatic hydrolysis processes.
Enzymatic hydrolysis processes aim at obtaining a high degree of hydrolysis (DH), and this is usually attained using a complex of unspecific acting proteolytic enzymes (i.e., unspecific acting endo- and exo-peptidases). For example, WO 94/25580 describes a method for hydrolyzing proteins by use of an unspecific acting enzyme preparation obtained from Aspergillus oryzae. Specific acting proteolytic enzymes have not been used for this purpose because such enzymes only lead to an inadequate degree of hydrolysis.
Acid carboxypeptidases (EC 3.4.16) are serine exopeptidases which catalyze the removal of amino acids from the C-terminus of peptides, oligopeptides or proteins. These carboxypeptidases generally have a narrow substrate specificity, i.e., they can cleave only few amino acids.
Acid carboxypeptidases of Aspergillus oryzae have been reported previously. For instance, Nakadai, Nasuno, and Iguchi, 1972, Agricultural and Biological Chemistry 36: 1343–1352, disclose a carboxypeptidase I with a molecular weight of 120 kDa (gel filtration) and optimal activity in the pH range 3.0 to 4.0. Nakadai, Nasuno, and Iguchi, 1972, Agricultural and Biological Chemistry 36: 1473–1480, disclose a carboxypeptidase II with a molecular weight of 105 kDa (gel filtration) and optimal activity at pH 3.0. Nakadai, Nasuno, and Iguchi, 1972, Agricultural and Biological Chemistry 36: 1481–1488, disclose a carboxypeptidase III with a molecular weight of 61 kDa (gel filtration) and a pH optimum of 3.0. Nakadai, Nasuno, and Iguchi, 1972, Agricultural and Biological Chemistry 37:1237–1251, disclose a carboxypeptidase IV with a molecular weight of 43 kDa (gel filtration) and optimal activity at pH 3.0. Tekeuchi and Ichishima, 1986, Agricultural and Biological Chemistry 50: 633–638, disclose a carboxypeptidase O with a molecular weight of 73 kDa (SDS-PAGE). Tekeuchi, Ushijima, and Ichishima, 1982, Current Microbiology 7: 19–23, disclose a carboxypeptidase O-1 and a carboxypeptidase O-2 both with a molecular weight of 63 kDa (gel filtration) and optimal activity at a pH in the range of 3.7 to 4.0. Ichishima et al., 1972, Journal of Biochemistry 72:1045–1048, disclose a comparison of the enzymatic properties of several Aspergillus acid carboxypeptidases. Azarenkova et al., 1976, Biokhimiya 41: 20–27, disclose the isolation of an acid carboxypeptidase from Aspergillus oryzae with a molecular weight of 37 kDa (SDS-PAGE) and a pH optimum of 4 to 5.
The production of protein hydrolysates with desirable organoleptic properties and high degrees of hydrolysis generally requires the use of a mixture of peptidase activities. It would be desirable to provide a single component peptidase enzyme which has activity useful for improving the organoleptic properties and degree of hydrolysis of protein hydrolysates used in food products either alone or in combination with other enzymes.
It is an object of the present invention to provide improved polypeptides having carboxypeptidase activity as well as methods of obtaining protein hydrolysates with desirable organoleptic qualities and high degrees of hydrolysis.